Add CSS variable to make the UI gaps consistent and fix the resize handle position (#7234)

Co-authored-by: J. Ryan Stinnett <jryans@gmail.com>

res/css/structures/_RoomView.scss

@@ -166,6 +166,7 @@ limitations under the License.
     flex: 1;
     display: flex;
     flex-direction: column;
+    margin-right: calc($container-gap-width / 2);
 }
 
 .mx_RoomView_statusArea {